Magnetotactic Bacteria at Cal Poly

Magnetotactic behavior in bacteria was discovered over 30 years ago by
microbiologist Richard P. Blakemore. The discovery was based on the fact that
certain motile, aquatic bacteria orient and migrate along magnetic field lines
when subjected to a magnetic field of the order of the geomagnetic field, or
greater. For magnetotactic bacteria (mtb) from Northern Hemisphere collection
sites, the predominant direction of migration in drops of water and sediment on
a microscope slide is parallel to the magnetic field corresponding to northward
migration in the geomagnetic field. The migration speed of individual bacteria
along the magnetic field lines depends on the field strength, but can be 90% or
more of the forward swimming speed of the cell (which can be up to 150 microns
per second). If the direction of the local magnetic field is reversed while mtb
are swiming, they execute a "U-turn" and continue migrating in the
same direction relative to the local magnetic field. The predominant migration
direction of mtb in the magnetic field can be reversed by subjecting the cells
to a strong (several hundred gauss) magnetic field pulse, oriented opposite to
the ambient field. Mtb that spontaneously migrate southward along geomagnetic
field lines are found in aquatic sediments and waters from the Southern
Hemisphere.

Magnetosome Mineral Particles

All mtb contain
magnetosomes, which are nanometer-sized, magnetic, mineral crystals enclosed in
a membrane. In most cases the magnetosomes are arranged in a chain or chains,
apparently fixed within the cell adjacent to the plasma membrane. In many mtb strains,
the magnetosome crystals are magnetite, Fe3O4,
characterized by a narrow size distribution and uniform, species-specific,
crystal habits. The crystal sizes typically range from ca. 40 to 100 nm, which
are within the permanent, single-magnetic-domain size range for magnetite.

In mtb from marine, sulfidic environments, the magnetosome crystals are the
iron-sulfide mineral greigite, Fe3S4, which is
isostructural with magnetite and is also ferrimagnetically ordered at room
temperature. The greigite crystals are also characterized by a narrow size
distribution and species-specific crystal habits. However, whereas the
magnetite crystals in a magnetosome chain are usually oriented so that a [111]
crystallographic axis of each particle lies along the chain direction, the
greigite particles in a magnetosome chain are usually oriented so that a [100]
crystallographic axis of each particle is oriented along the chain direction.
While most mtb strains have either magnetite or greigite magnetosomes, there is
an organism which has both magnetite and greigite magnetosomes co-organized in
chains.

Constructing the Cellular Magnetic Dipole

For cells with
either magnetite or greigite magnetosomes, the chain of magnetosomes
constitutes a permanent magnetic dipole fixed within the bacterium. The
magnetic dipole moment is generally sufficiently large so that it, and
consequently the bacterium, is oriented in the geomagnetic field at ambient
temperatures. Thus magnetotaxis is a passive process in which the orientation
of the magnetic dipole in the ambient magnetic field as the organism swims
causes it to migrate along the magnetic field lines. Killed cells align along
the field but do not migrate. Thus motile mtb behave like self-propelled,
magnetic, compass needles.

Mtb have two possible magnetic polarities, depending on the orientation of the
magnetic dipole within the cell. The polarity can be reversed by a magnetic
pulse which is greater than the coercive force of the chain of particles
(several hundred gauss). Bacteria with reversed polarity migrate along magnetic
field lines in the direction opposite to that of bacteria with the original
polarity. In natural habitats, the predominant polarity type in a population of
a given bacterial species is determined by the sign of the inclination of the
geomagnetic field.

Chemically Stratified Sites

It has been reported that high concentrations of mtb occur in
a horizontal "plate" at the oxic-anoxic transition zone (OATZ) in chemically
stratified marine environments. In these environments, downward oxygen
diffusion from the surface and upward sulfide diffusion, resulting from
bacterial sulfate reduction in the anaerobic sediment, create a double vertical
chemical concentration gradient system, with a concomitent redox gradient. Salt
Pond, a 5 meter deep coastal pond in Falmouth, MA on Cape Cod, stratifies in
the summer with the OATZ at about 3 meters. The mtb concentration in the plate
is greater than 105 cells per cc. At least seven
morphologically-distinct, magnetotactic-bacterial types occur at the OATZ, some
containing Fe3O4 particles, and some containing Fe3S4 particles [21]. The
magnetite-containing cells tend to be more abundant at the top of the plate, in
the relatively oxygen-rich portion of the OATZ, while the greigite-containing
cells tend to be more abundant at the bottom of the plate, in the relatively
sulfide-rich portion of the OATZ.